MEXICO CITY — Rescue workers searched the rubble for survivors and authorities promised an investigation after an explosion killed at least 25 people and injured 101 at the headquarters of Mexico’s state-owned oil company, Petróleos Mexicanos, or Pemex.

The cause of the basement explosion in an administrative building next to the iconic, 51-story Pemex tower in Mexico City remained a mystery early Friday, with President Enrique Peña Nieto urging people not to speculate. Theories ranged from an electrical fire to an air-conditioning problem to a possible attack.

“We have no conclusive report on the reason,” Peña Nieto said. “We will work to get to the bottom of the investigation to find out, first, what happened work, and if there are people responsible in this case, that we apply the full weight of the law against them.”

Some 46 people remained hospitalized after the Thursday afternoon blast, some gravely injured and others with cuts, fractures and burns. Authorities said the dead were 17 women and eight men.

More than 500 firefighters, soldiers and rescue workers dug through chunks of concrete with dogs, trucks and a Pemex crane at the five-building Pemex complex.

Interior Minister Miguel Osorio Chong said it was uncertain if any of the roughly 10,000 people who work in the five-building headquarters were still trapped, but the search would continue. The explosion occurred at about 3:45 p.m., just as the administrative shift was about to end. It hit the basement and first two floors, which rescuers said all collapsed onto each other.

Pemex first said it had evacuated the tower and 14-story administrative building because of a problem with the electrical system. The company later tweeted that the Attorney General’s Office was investigating the explosion.

Ana Vargas Palacio was distraught as she searched for her missing husband, Daniel Garcia, 36, who works in the building where the explosion occurred. She said she last talked to him a couple hours earlier.

“I called his phone many times, but a young man answered and told me he found the phone in the debris,” Vargas said. The two have an 11-year-old daughter.

Gabriela Espinoza, 50, a Pemex secretary for 29 years, was on the second floor of the tower when she said she heard two loud explosions and a third smaller one.

“There was a very loud roar. It was very ugly,” she said.

“We were talking and all of sudden we heard an explosion with white smoke and glass falling from the windows,” said Maria Concepcion Andrade, 42, who lives on the same block as the Pemex building. “People started running from the building covered in dust. A lot of pieces were flying.”

Pemex, created as a state-owned company in 1938, has nearly 150,000 employees and in 2011 produced about 2.5 million barrels of crude oil a day, according to its website, with $111 billion in sales.